Control system for clothes dryers



Nov. 21, 1961 Filed July 9, 1959 D. W. LYNCH CONTROL SYSTEM FOR CLOTHESDRYERS 3 Sheets-Sheet 1 FIGJ INVENTOR. DONALD w. LYNCH BY 1L2 f imm,

HIS ATTORNEY Nov. 21, 1961 D. w. LYNCH CONTROL SYSTEM FOR CLOTHES DRYERSFiled July 9, 1959 3 Sheets-Sheet 2 lZn.

INVENTOR.

D NALD W. LYNCH BY w ,0 4 I v HIS ATTORNEY Nov. 21, 1961 D. w. LYNCHCONTROL SYSTEM FOR CLOTHES DRYERS 5 Sheets-Sheet 3 Filed July 9, 1959 79AMBIENT TEMPERATURE COMPENSATING CONTROL PIC-1.3

INVENTOR LYN c H H l 5 ATTORNEY United States Patent 3,009,256 CONTROLSYSTEM FOR CLOTHES DRYERS Donald W. Lynch, Louisville, Ky., assignor toGeneral Electric Company, a corporation of New York Filed Juiy 9, 1959,Ser. No. 825,976 8 Claims. (Cl. 34--45) My invention relates to clothesdrying machines, and more particularly to automatic control systems foruse in such machines for controlling the drying operation.

There are at the present time several domestic clothes drying machinescommercially available where the length of the drying cycle iscontrolled indirectly by the dryness of the clothes. This is generallyeffected by using a thermostatic control, on the basis that apredetermined rise in the clothes temperature indicates that the heatsupplied to the clothes is no longer being used to vaporize moisturefrom them but instead is free to raise their temperature. In otherwords, such control systems utilize a predetermined high temperature asa trip point for shutting off the means provided for heating theclothes. Depending upon other structural features of the differentdrying machines, a single such occurrence may be provided or severalsuch occurrences may be provided in a single drying sequence. In eitherevent, it is important for optimum operation that suitable changes bemade to the trip temperature at which the heaters are shut off tocompensate for variations in the ambient temperature. This results fromthe fact that when the temperature of the air taken in for circulationover the heaters and through the clothes container is quite low, itatfects substantially the temperature to which the heaters can raise theair before it enters the basket. Since the air flow has a substantialefiect on the drying of the clothes as well as the heat, variations inthe temperature of the air passing through the basket will not, providedthey are fairly small, have too substantial an effect on the length oftime it takes to dry the clothes. However, where a thermostat is socalibrated that it trips when clothes are dry and the air being taken inis at an ambient temperature of 70 degrees Fahrenheit, for instance, itwill readily be understood that should the ambient temperature drop to50 degrees a much greater temperature rise will be necessary for thethermostat to be tripped. This has the result, if not com pensated for,of excessively lengthening the drying cycle to the extent that theclothes are dry long before the end of the cycle. In fact, if thetemperature drop is sufliciently great-as may be the case where a dryeris kept in an unheated basement or garage-the dryer may continue tooperate without shutting off until manually turned to its off position.Because of this it is of high importance to provide suitablecompensating means responsive to the temperature of the ambient airbeing drawn into the dryer.

It is therefore an object of this invention to provide improvedeconomical means of compensating for the ambient temperature changes ina dryer having a thermostatic control of the general type described.

It is further an object of the invention to achieve this compensation bysuitable variation of [the heating effect of a biasing heater positionedin heating relation to the main thermostat.

Yet a further object of the invention is to achieve this variation ofthe heating effect of the biasing heater by providing diiferentresistances in parallel with each other and in series with the biasingheater, with means sensitive to ambient temperature for opening thecircuits to diiferent resistances at different ambient temperatures.

A further general object of the invention is to vary the heating elfectof the biasing heater by providing at least two parallel paths with aswitch in one of them so that when the switch is open a first resistanceis provided and Patented Nov. 21, 1961 when the switch is closed asecond resistance is provided, and to operate the switch in response tothe occurrence of a predetermined condition such as, for instance, theabove specifically described ambient temperature condition.

In carrying out my invention in one form thereof, 1 provide a clothesdrying machine which has a conventional clothes container with a mainheater positioned to heat the clothes in the container. Thermostaticmeans are positioned to be responsive to the temperature of the clothesin the container and are also acted on by an electric resistance-typebiasing heater which is positioned in suitable heating relation to thethermostatic means. When the thermostatic means senses a predeterminedhigh temperature, due to the combination of the clothes temperature andthe biasing heater, it causes a first switch to open to stop operationof the main heater. As thus far described, it is clear that the moreeffective the biasing heater, the lower the clothes temperature whichwill trip the thermostatic means.

In series with the biasing heater we provide at least two conductorsconnected in parallel with each other. One of the two conductorsincludes a switch, and the resistances of the conductors are soproportioned that a first resistance in series with the biasing heateris pro vided with the second switch open and a second difierentresistance when the second switch is closed. Means are providedresponsive to the occurrence of a predetermined condition (such as arise in the ambient temperature to a predetermined level) toautomatically open the second switch; when this occurs, the relativelylow resistance path of energization for the biasing heater is changed toa relatively high resistance path, and the biasing heater mustconsequently be energized through a circuit which allows less power thanbefore to be used in the heater and therefore decreases itseffectiveness.

With the heat output of the biasing heater thus decreased, the clothestemperature required to trip the main thermostatic means must then behigher. This rise in the clothes temperature may be made tosubstantially oifset the rise in the ambient temperature.

The subject matter which I regard as my invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. My invention, however, both as to organization and methodof operation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconjunction with the drawings.

In the drawings,

FIGURE 1 is a side elevational view of a clothes dryer incorporating myimproved dryer control arrangement, the view being partly broken awayand partly sectionalized to illustrate details;

FIGURE 2 is a horizontal sectional view of the dryer, with certainsurfaces broken away and partly sectionalized to illustrate furtherdetails; and

FIGURE 3 is a schematic illustration of my improved control circuit.

Referring now to FIGURES 1 and 2 of the drawings, I have shown therein adomestic clothes dryer 1 including a clothes tumbling container or drum2 provided within a suitable outer casing or cabinet 3 which completelyencloses it on all sides. The drum is mounted for rotation within acabinet 3 on a substantially horizontal axis, and is generallycylindrical in shape, having a first outer wall portion 4, second andthird outer wall portions 5 and 6 located respectively adjacent the endsof the drum, a back wall 7 and a front wall 8. Outer wall portions 4, 5and 6 are imperforate over their entire length, so that the entire outershell of the basket is imperforate; on the interior surface of centralportion 4 accompanying there is provided a plurality of clothes tumblingribs 9.

The front of drum 2 is rotatably supported within the outer casing 3 bymeans of a pair of idler roller wheels and 11 which are rotatablysecured to the top of an upwardly extending member 12 secured at itsbottom to the base 12a of the machine (FIGURE 1). It will be observedthat rollers 10 and 11 are disposed beneath the drum in contact withportion 6 thereof. The rear end of the drum receives its support from aroller wheel 13 which is positioned beneath portion 5 of the drum insupporting and frictionally driving engagement therewith.

Roller 13 is mounted on a shaft 14 supported in bearings 15. Shaft 14 issecured to a pulley 16 which is driven from a belt 17 in turn powered bya pulley 18 mounted on the end of a shaft 19 of an electric motor 20.The motor, pulleys, and roller 13 are so proportioned to drum 2 and toeach other that drum 2 is rotated by roller 13 at an appropriate speedto provide tumbling action for articles of clothing placed therein.

In order that a stream of drying air may be introduced into and passedthrough the clothes drum, the drum is provided with a central aperture21 in its front wall 8 and with a plurality of perforations 22 extendingin an annulus around back wall 7. Rigidly secured to the rear wall 23 ofcasing 3 by any desired means such as, for instance, welding at suitablepoints 24, is a baffle member 25 which has secured thereto heating means26 appropriately insulated from the baffle member. Heating elements 26may be annular in shape so as to be generally coextensive with theperforations 22 in drum 2. A bafile member 27 is rigidly secured to theback wall 7 of the drum outside the ring of perforations 22 and withinthe stationary baffie 25 so that an annular air inlet 28 is, in effect,formed by the two bafiles 25 and 27. Battle 27 is further provided withan annular series of openings 29; in this manner a passage is providedfor air to enter annular opening 28 between the bafiles, pass over theheating elements 26, through openings 29 and then through perforations22 into the interior of drum member 2.

In addition to the air guiding function, the battles 25 and 29 help therollers 10, 11 and 13 support the drum 2. Secured to the central portion30 of bathe 27 is a bracket 31 to which in turn is secured a stub shaftmember 32 arranged substantially coaxial with drum 2. The centralportion 30 of baffle 27 has an opening 34 formed therein in the form ofa slot. The slot is suitably formed (as more fully described inapplication S.N. 789,869 filed January 29, 1959, by Willam F. Simpsonand assigned to the same assignee as this invention) so as to permitstub shaft 32 a limited amount of movement in the vertical direction but'virtually no movement in a horizontal direction. Thus, the slightvertical motions of the drum which result from the tumbling of theclothes can be accommodated while at the same time undesirablehorizontal motion is afiirmatively prevented by the engagement of stubshaft 32 in slot 34.

The front opening 21 of the drum is substantially closed by means of astationary bulkhead generally indicated by the numeral 35. Bulkhead 35is made up of a number of adjacent members including the inner surface36 of an access door 37 mounted on the dryer cabinet, a stationary frame38 for the door, the inner surface 39 of an exhaust duct 40, and anannular flange 41 mounted on the frame 38 and on the duct wall. It willbe noted that a suitable clearance is provided between the inner edge ofthe drum opening 21 and the edge of bulkhead 35 so that there is norubbing between the drum and the bulkhead during rotation of the drum.In order to prevent any substantial air leakage through opening 21between the interior and the exterior of the drum a suitable ring seal42, preferably formed of felt-like material, is secured to flange 41 insealing relationship with the exterior surface of drum wall 8.

Front opening 21, in addition to serving as part of the air flow paththrough the drum, also serves as a means whereby clothes may be loadedinto and unloaded from the drum. The door 37, whose inner surface formspart of the bulkhead closing the opening, is mounted on cabinet 3 andwhen the door is opened clothes may be inserted into or removed from thedrum through the door frame 33. It will be noted that the door includesan outer fiat imperforate section 43 and an inwardly extending hollowsection 44 mounted on the fiat outer section. Hollow section 44 extendsinto the door frame 38 when the door is closed, and the door surface 36which comprises part of the combination bulkhead 35 is actually theinner wall of this hollow section.

The air outlet from the basket is provided by a perforated opening 45formed in the inner wall 36 of hollow door section 44. The bottom wallsection of door 34 and the adjacent wall of door frame 38 are providedwith aligned openings 46 and 47, opening 47 providing the entrance toduct 40. As shown, a lint trap 48, which may comprise a fine mesh bag,is preferably positioned in exhaust duct 40 at opening 47, the bag beingsupported by the door frame 38.

Duct 40 leads to suitable air moving means which may, as shown, comprisea centrifugal blower 49 mounted on the motor shaft and thus driven bymotor 20. The outlet of blower 49 communicates with an outlet duct 50(FIG- URE 2) which extends to an opening 51 in the back 23 of cabinet 3.During operation of motor 20, the rotation of blower 49 causes air to bedrawn into cabinet 3 through a suitable opening 51a, through annularopening 28, over heaters 26, through openings 29 and perforations 22into drum 2, across the drum, through perforated opening 45 and alignedopenings 46 and 47 into duct 40, and then through the blower 49, theoutlet duct 50, and opening 51 to atmosphere.

The operation of the dryer 1 is controlled by a new and improved controlsystem, one embodiment of which is shown in the circuit diagram ofFIGURE 3. As shown there, the entire control system of the machine isenergized across a three wire power supply system including supplyconductors 52 and 53 and a neutral conductor 54. For domestic use, theconductors 52 and 53 will normally be connected across a 220 volt powersupply with volts appearing between the neutral line 54 and each ofthese conductors.

The drive motor 20 includes a main winding 55 and a start winding 56which are connected in parallel with each other between supply conductor52 and neutral conductor 54. Specifically, the circuit, starting asupply conductor 52, passes through a switch 57, then through thewindings 55 and 56 in parallel with each other, a standard motorprotector device 58, a switch 59 positioned (FIGURE 1) to be opened whenthe door 37 is opened and closed when the door is closed, and neutralconductor 54. Included in the parallel branch circuit of auxiliarywinding 56 is a switch 60 controlled by a centrifugal mechanism 61rotatable with the motor; when the motor comes up to a certain speedupon energization of the windings 55 and 56, the device 61 will causeswitch 60 to open thereby de-energizing winding 56 and permitting themotor to continue running on winding 55 alone.

Returning to switch 57 which is in series with both windings of themotor, this switch is under the control of a cam member 62 which in turnis driven by a timer motor 63 when energized. The timer motor alsocontrols a cam member 64 which operates a switch 65. As well as beingdriven by the timer motor 63 when energized, the cams 62 and 64 aremanually presettable to a desired position by means of a control member66 which is generally provided in a readily accessible position such as,for instance, as shown in FIGURE 1, on the backsplasher 67 secured tothe top of cabinet 3. As also shown in FIGURE 1, the timer motor 63 andthe cam and switches controlled thereby form together a control assembly68 which is generally positioned within backsplasher 67 as shown. Theenergizing circuit for timer motor 63, starting with supply conductor52, passes through switch 65, a switch 69 controlled by centrifugalmechanism 61 and closed thereby at the same time that switch 60 isopened, a main thermostatic device 70, preferably in the form of abimetal element as shown, and timer motor 63 to conductor 54. Thebimetal thermostatic means has a high temperature trip position in whichit moves into engagement with a contact 7i]. and a low temperature resetposition in which it moves into engagement with a contact 72. Thecircuit through the timer motor 63 is completed when element 70 engagescontact 71. Alternatively, when element 70 engages contact 72, a circuitis completed, starting at supply conductor 52, through switches 65 and69, bimeta l element 70, contact 72, protective device 73, and theheaters 26 to conductor 53.

It will be observed in FIGURE 1 that the thermostatic device 70 ispositioned at the front of the dryer onthe bulkhead 35 directly belowthe air outlet 45 from the drum. The thermostatic element 70 is thuseffective to sense the temperature of the exhaust air leaving the drum,which temperature is very close to the actual clothes temperature. Inaddition to the clothes temperature as a factor affecting the operationof thermostatic means 70, a biasing heater 74 is provided in closephysical relation ship with element 70 so as to be in heating relationthereto. The heater 74 is of relatively high resistance compared toheater 26 (for instance approximately 1100 ohms for heater 74 and 10ohms for heater 26 may be pro vided), so that its power consumptioncompared to the main heater is exceedingly small. In fact, the smallsize of heater 74 and the desirability of having it close to thethermostatic element 70 generally causes the preferred construction, inpractice, to provide for a unitary assembly within a single casing,which assembly includes both the biasing heater 74 and the thermostaticelement 70.

The biasing heater 74 is energized through a circuit which, starting atconductor 52, extends through a group 79 of parallel connectedconductors, the biasing heater 74, and neutral conductor 54, the groupconstituting an ambient temperature compensating control and beingpositioned in any suitable location (such as backsplasher 67) Where itmay sense the ambient temperature.

Group 79 includes three conductors 80, 81 and 82. While any combinationof suitable resistances may be provided for the purpose to be explaineddirectly herebelow, I provide as one means of carrying out my inventiona relatively high resistance 83 in conductor 80, a smaller resistance 84in conductor 81, and a very small resistance, which may be that inherentin the wire itself, in the third conductor 82. In series with theresistances in conductors 81 and 82 are a pair of thermostaticallyresponsive devices 85 and 86 respectively which may, as shown, be simplysmall bimetallic cantilever switch members. Thermostats 85 and 86 arepositioned, as shown in FIGURE 2, to be responsive to the ambienttemperature, that is, to the temperature of the air surrounding thedryer which will be drawn into the dryer by the blower for heating andcirculation through drum 2. Thermostats 85 and 86 are calibrated so thatthey each close at a different ambient temperature; the thermostat 85closes at the higher temperature (on decreasing ambient temperatures),preferably on the order of 90 F. White theother thermostat 86 closes ata substantially lower ambient temperature, which may be on the order of70.

It will thus be seen that if the temperature is above 90 boththermostats 85 and 86 will be opened so that the circuit for the biasingheater has to pass through the relatively high resistance 83. Theprovision of a relatively to be supplied to the heater with the resultthat a greater amount of heat from the clothes in the drum 2 is requiredbefore thermostat 70 trips. Thus, a relatively high ambient temperaturecauses a relatively high dlothes temperature to be required before thethermostat 70 trips. When the temperature is between 90 and 70, thethermostat closes, and a smaller resistance 84 is in the circuit inseries with biasing heater 74 so that the heater is more effective and alower temperature from the clothes in the drum is required to tripthermostat 70. Thus, a lowering of the ambient temperature causes alowering of the clothes temperature required to trip the thermostat 70.In similar fashion, when the ambient temperature is below 70, thermostat"86 closes to provide a very low resistance path in series with biasingheater '74, the biasing heater is fully effective, and a relatively lowtemperature of the clothes causes tripping of the thermostat 70.

' For a full understanding of the invention, let it now be assumed thata load of ordinary cottons is to be dried, and that the machine islocated in a room where the temperature of the air to be drawn into themachine is 75. The dial 66 is appropriately marked so that when thetimer is set to run for a relatively long period it indicates that aheavy load of moisture-absorbent clothes is to be dried, for anintermediate period it indicates that ordinary cottons are to be dried,and for short periods it indicates that synthetics are to be dried. Withan average type load such as ordinary cottons, the dial 66 is set sothat approximately /3 of the time available is provided. This closesswitches 57 and 65; also, with the temperature 75, thermostat 86 isopened and thermostat 85 is closed. In addition, since the cycle is juststarting, the main thermostat 70' is in its cool or re-set positionwhere it engages contact 72 to provide an energizing circuit for theheaters 26 and to prevent the timer motor 63 from operating.

Provided the door is closed so that switch 59 is closed, drive motor 20will then start in operation. When it has come up to speed, the startwinding 56 is de-energized and switch 69 is closed by centrifugalmechanism 61. The closing of switch 69 completes an energizing circuitfor heaters 26 which thereupon start to heat the air as it is drawn intothe drum 2 by the blower 49.

At the same time, the biasing heater 74 is energized in series with theparallel resistance 83 and 84. The combination of resistances 83 and 84is that which is selected for connection in the circuit when the ambienttemperature is between 70 and that is, when thermostat 85 is closed andwhen thermostat 86 is opened. The inclusion of resistance 84 in thecircuit by the closing of thermostat 85 has the desired compensatingeffect onbiasing heater 74. The condition of the thermostats 8-5 and 86is not, of course, affected by the progression of the drying cycle, andif as is normally true the ambient temperature remains constant within afew degrees during the drying cycle, the combination of resistances 8'3and 84 will continue to determine the ambient temperature compensationeffect on biasing heater 74.

As stated, the heaters 26 are energized; after a period of suchenergization, and in accordance with the effect of biasing heater 74 asmodified by group 79, the thermostat 70 will trip in response to a risein the clothes tem perature and will engage contact 71. Thisde-energizes heaters 26 and energizes timer motor 63 which thencontinues to operate until such time as the thermostatic element 70resets and engages contact 72. At this point, of course, the heaters 26are re-energized and the timer motor 63 is de-energized. This causes thetemperature to rise again, and when the predetermined point is againreached the thermostat again causes de-energization of the heater andre-energization of the timer. It will be seen that this sequence willcontinue until such time as the timer has caused the cams to rotate tothe off position high resistance in series with heater 74 causes lesspowerat which point all circuits within the dryer will be opened 7 andthe sequence will be terminated, with the clothes within the drum beingcompletely dry.

It will thus be observed that a drying cycle wherein the cycle isdependent to a substantial extent on the tripping of a thermostatsensitive to the clothes temperature has been provided which in aneconomical manner provides for compensation for the ambient temperature.It will readily be seen that if, with the ordinary cotton load, thetemperature should have been, for instance, 60, thermostat 86 would haveclosed and a substantially lower clothes temperature would have causedtripping; by the same token, a high temperature above 90 would havecaused opening of both thermostats 85 and 86 and the clothes temperaturerequired to trip thermostat 7 would also have been raised.

It will thus be seen that an economical and effective systemis providedfor tailoring the operation of the clothes drying machine 1 to theoccurrence or variation of a particular condition such as, for instance,the ambient temperature. It will be recognized that while a particularautomatic sequence has been described, in which the operation of theheater and the timer are completely alternative to each other, myinvention is of value in other types of drying cycles, including thosewhere operation of the heaters and timer is concurrent part of the time,and even those where the operation of the timer is continuous; in fact,the ambient compensation feature has its usefulness unimpaired even ifall timing means be eliminated and the thermostatic means provide thesole control.

It will further be recognized that the biasing heater may be made tooperate continually as shown, or that its operation may be contingent onsome other factor such as the operation of the main heaters 26. In thepresent embodiment it is deemed preferable that the operation of thebiasing heater be continuous, but it is readily conceivable that in somecases, particularly where the operation of the heaters and the timemotor is not completely alternative in nature, it might be desirable tohave the biasing heater operate at the same time as the main heaters.

Another point to be noted is that while each group of conductors hasbeen shown to include three conductors, it is readily apparent that asfew as two and as many as desired may be provided without departure fromour invention. Also, while conductors of varying resistance have beenshown, it will readily occur that this arrangement is not necessarilythe only one which may be effective. For instance, if equal resistancesare placed in two parallel conductors, it is well known that if thecircuit through one of the conductors is opened the resistance in thecircuit is then twice as large as when the connections are completedthrough the two parallel conductors. Thus, far from being limited fromdifferent resistors in each conductor it will readily be apparent thatthe differing resistance effect may be obtained even with likeresistors. Also, a compensating resistance change is obtainable with theswitch means in series with the higher resistance, particularly wherethe lower resistance is still fairly substantial (as opposed to a shortcircuit).

While in accordance with the patent statutes I have described what atpresent is considered to be the preferred embodiment of my invention, itwill be obvious to those skilled in the art that various changes andmodifications may be made therein without departing from the invention,and it is therefore aimed in the appended claims to cover all suchequivalent variations as fall within the true spirit and scope of theinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. In a clothes drying machine, a clothes container, a main heaterpositioned to heat clothes in said container, first thermostatic meanspositioned to be responsive to the temperature of clothes in saidcontainer, an electric resistance type biasing heater positioned inheating relation to said first thermostatic means, first switch meanscontrolling operation of said main heater, said first thermostatic meanscausing said first switch means to open in response to a predeterminedhigh temperature, at least two parallel connected conductors in serieswith said biasing heater, each of said conductors having a differentresistance included therein, the one of said conductors including thelesser of said resistances further including a second switch means inseries with said lesser resistance, and second thermostatic meanspositioned to be responsive to the ambient temperature, said secondthermostatic means causing opening and closing of said second switchmeans when the ambient temperature rises above and falls below apredetermined level, whereby the effect of said biasing heater on saidfirst thermostatic means is modified and the clothes temperaturerequired to cause said first thermostatic means to open said firstswitch means is increased and decreased with increases and decreases inthe ambient temperature.

2. In a clothes drying machine, a clothes container, a main heaterpositioned to heat clothes in said container, first thermostatic meanspositioned to be responsive to the temperature of clothes in saidcontainer, an electric resistance type biasing heater positioned inheating relation to said thermostatic means, first switch meanscontrolling operation of said main heater, said first thermostatic meanscausing said first switch means to open in response to a predeterminedhigh temperature, at least two parallel connected conductors in serieswith said biasing heater, at least one resistor and at least one secondswitch means connected in said conductors, said conductors providing agreater resistance when said second switch means is open and a lesserresistance when said second switch means is closed, second thermostaticmeans positioned to be responsive to the ambient temperature, saidsecond thermostatic means causing opening and closing of said secondswitch means when the ambient temperature rises above and falls below apredetermined level, whereby the effect of said biasing heater on saidfirst thermostatic means is modified and the clothes temperaturerequired to cause said first thermostatic means to open said firstswitch means is increased and decreased with increases and decreases inthe ambient temperature.

3. The apparatus defined in claim 2 wherein said first thermostaticmeans comprises a bimetallic element.

4. In a clothes drying machine, a clothes container, a main heater, aircirculating means arranged to circulate ambient air over said heater andthen into said container, first thermostatic means positioned to beresponsive to the temperature of clothes in said container, an electricresistance type biasing heater positioned in heating relation to saidfirst thermostatic means, first switch means controlling operation ofsaid main heater, said first thermostatic means causing said firstswitch means to open in response to a predetermined high temperature, atleast two paralle connected conductors in series with said biasingheater, at least one resistor and at least one second switch meansconnected in said conductors, said conductors providing a. greaterresistance when said second switch means is open and a lesser resistancewhen said second means is closed, and second thermostatic meanspositioned to be responsive to the temperature of the ambient air, saidsecond thermostatic means controlling said second switch means andopening said second switch means when the ambient air temperature risesto a predetermined level whereby variations in the temperature of theair circulated over said main heater and into said container arecompensated for by variations in the effect of said biasing heater onsaid first thermostatic means.

5. In a clothes drying machine, a clothes container, a main heater,control means for limiting the length of operation of said machine,manually operable means for presetting the duration of control meansoperation, air circulating means arranged to circulate ambient air oversaid main heater and then into said clothes container, firstthermostatic means positioned to be responsive to the temperature ofclothes in said container, an electric resistance type biasing heaterpositioned in heating relation to said first thermostatic means, firstswitch means controlling operation of said main heater and of saidcontrol means, said first thermostatic means causing said first switchmeans to de-energize said main heater and energize said control meansupon sensing a predetermined high temperature, and causing said firstswitch means to energize said main heater and de-energize said timerupon sensing a predetermined low temperature, at least two parallelconnected conductors in series with said biasing heater, at least oneresistor and at least one second switch means connected in saidconductors, said conductors providing a greater resistance when saidsecond switch means is open and a lesser resistance when said secondswitch means is closed, and second thermostatic means responsive to thetemperature of the ambient air, said second thermostatic meanscontrolling said second switch means and opening said second switchmeans when the ambient air temperature rises to a predetermined level.

6. The apparatus defined in claim 2 wherein said biasing resistance isconnected to operate continuously during a drying operation.

7. In a clothes drying machine, a clothes container, a main heaterpositioned to heat ciothes in said container, first thermostatic meanspositioned to be responsive to the temperature of clothes in saidcontainer, an electric resistance type biasing heater positioned inheating relation to said first thermostatic means, means controllingoperation of said main heater, said first thermostatic means causingsaid controlling means to shut off said main heater in response to apredetermined high temperature, at least two parallel connectedconductors in series with said biasing heater, at least one resistor andat least one switch means connected in said conductors, said conductorsproviding a greater resistance when said switch means is open and alesser resistance when said switch means is closed, and secondthermostatic means positioned to be responsive to the ambienttemperature, said second thermostatic means causing opening and closingof said one switch means when the ambient temperature rises above andfalls below a predetermined level, whereby the effect of said biasingheater on said first thermostatic means is modified and the clothestemperature required to cause said first thermostatic means to shut ofisaid main heater is increased and decreased with increases and decreasesin the ambient temperature.

8. In a clothes drying machine, a clothes container, a main heaterpositioned to heat clothes in said container, first thermostatic meanspositioned to be responsive to the temperature of clothes in saidcontainer, an electric resistance type biasing heater positioned inheating relation to said thermostatic means, means controlling operationof said main heater, said first thermostatic means causing saidcontrolling means to shut ofi said main heater in response to apredetermined high temperature, at least two parallel connectedconductors in series with said biasing heater, at least one resistor andat least one switch means connected in said conductors, said conductorsproviding a greater resistance when said switch means is open and alesser resistance when said switch means is closed, second thermostaticmeans positioned to be responsive to the ambient temperature, saidsecond thermostatic means causing opening and closing of said switchmeans when the ambient temperature rises above and falls below apredetermined level, whereby the etfect of said biasing heater on saidfirst thermostatic means is modified and the clothes temperaturerequired to cause said controlling means to shut oif said main heater isincreased and decreased with increases and decreases in the ambienttemperature.

References Cited in the file of this patent UNITED STATES PATENTS2,775,047 Morrison Dec. 25, 1956 2,851,790 Worst Sept. 16, 19582,878,579 Fuchs Mar. 24, 1959

